It is desirable that a mobile communication system can achieve high-speed, high-capacity, and low-latency. One method for achieving these is carrier aggregation (CA). When the CA is utilized, user equipment can use a plurality of carriers simultaneously. Each one of the plurality of carriers is referred to as a component carrier (CC). The individual CC corresponds to one system, such as a long-term evolution (LTE) mobile communication system or a 3G mobile communication system. The 3G mobile communication system includes, for example, mobile communication systems based on the W-CDMA scheme and the GSM scheme. The CA is used for an LTE-Advanced mobile communication system (cf., Non-Patent Document 1, for this point). Among a plurality of CCs that can be used by user equipment, one of the CCs is referred to as a main carrier or a primary component carrier (PCC), and the CCs other than the one of the CCs are referred to as secondary component carriers (SCC). Processing of communication with the PCC and the SCCs includes cell search, measurement, and monitoring of a radio link, as well as transmission and reception of control information and user data.
In contrast, user equipment satisfying a specific condition operates in a discontinuous reception (DRX) mode, so as to reduce battery energy consumption of the user equipment. For convenience of the explanation, it is assumed that there are two options for an operation mode of user equipment, which are a continuous reception mode and a discontinuous reception mode. For a case of a discontinuous reception mode, user equipment operates with a cycle that is referred to as a discontinuous reception cycle (DRX cycle). A mode of user equipment that operates in the discontinuous reception mode transitions to an active mode during a predetermined period (On-duration) per discontinuous reception cycle, and transitions to an inactive mode until the following cycle after elapse of the period. When user equipment that is operating in a continuous reception mode receives a downlink control signal from a base station, the user equipment activates a timer (drx-inactivity timer), and when a constant time period is elapsed without receiving a further downlink control signal by the user equipment, the mode of the user equipment transitions to a discontinuous reception mode. A specific example of the downlink control signal may be a physical downlink control channel (PDCCH) of an LTE mobile communication system. However, the downlink control signal is not limited to this. Here, a continuous reception mode is a mode other than a discontinuous reception mode. Note that a continuous reception mode includes, for example, a mode of receiving a downlink control signal for each subframe, and it is not necessarily that downlink control signals are continuously received. Such a discontinuous reception (DRX) control is described in Non-Patent Document 2.
It can be considered to execute DRX control during execution of carrier aggregation (CA). However, when DRX control according to related art is applied to the CA as it is, and when a constant time period is elapsed without receiving, for each of the CCs, a downlink control signal, the user equipment may transition to a discontinuous reception mode.
However, communication traffic for the component carriers of the plurality of component carriers (CCs) is not necessarily uniform. For example, traffic may hardly occur for a first CC, while a significant amount of traffic may occur in a second CC. If user equipment were unable to transition to a discontinuous reception mode unless no downlink control signal is received for a constant time period for the first CC and for the second CC, the user equipment would wastefully keep a signal processor for the first CC running. In such a case, battery energy may be consumed more than necessary. As described above, a problem of concern is that DRX control according to related art may not be efficient for a mobile communication system in which carrier aggregation (CA) can be executed.